1. Field of the Invention
The present invention relates to a magnetic detector for applying a bias magnetic field to the magnetic resistance element in detecting a multipole-magnetized moving body with a magnetic resistance element (MR element).
2. Description of the Related Art
For example, there is the system wherein electrodes are respectively formed in ends of each of magnetic resistance segments constituting a magnetic resistance element to construct a bridge circuit, and a power source with constant voltage and current is connected between the two opposite electrodes of the bridge circuit to convert a change in resistance value of the magnetic resistance segment into a change in voltage, thereby detecting a change in magnetic field acting on the magnetic resistance element.
A conventional magnetic detector will now be described with reference to the associated ones of the accompanying drawings. FIGS. 4A and 4B are respectively a perspective view and a plan view each showing a construction of the conventional magnetic detector.
In FIGS. 4A and 4B, reference numeral 1 designates a disc-like magnetic moving body having projections in its periphery and having a shape for changing a magnetic field; reference numeral 2 designates a processing circuit portion in which a circuit is printed on the surface of a board; reference numerals 2a and 2d designate respectively magnetic resistance segments; reference numerals 2b and 2c designate respectively magnetic resistance segments; reference numeral 3 designates a magnet; and reference numeral 4 designates a rotational axis of the magnetic moving body 1. The rotational axis 4 is rotated so that the magnetic moving body 1 is also rotated synchronously therewith. Incidentally, for example, the magnetic resistance segments 2a and 2d are illustrated by one black block because the individual segments are so compacted that one segment can not be illustrated independently.
FIG. 5 is a circuit diagram showing a construction of the processing circuit portion of the conventional magnetic detector employing a magnetic resistance element.
In FIG. 5, the magnetic resistance element is constituted by the magnetic resistance segments 2a to 2d. Also, in the figure, reference numeral 12 designates a differential amplification circuit, reference numeral 13 designates an A. C. coupling circuit, reference numeral 14 designates a comparison circuit, reference numeral 15 designates an output circuit, reference symbol 15T designates a transistor, and reference symbol 15Z designates an output terminal.
In FIG. 5, a constant voltage VCC is applied to the bridge circuit constituted by the magnetic resistance segments 2a to 2d or fixed resistors to convert the changes in resistance values of the magnetic resistance segments 2a to 2d due to the change in magnetic field into a voltage change. The signal which has been obtained by the conversion into the voltage change is amplified by the amplification circuit 12 to be inputted to the comparison circuit 14 through the A. C. coupling circuit 13. The signal a level of which has been compared with a predetermined voltage by the comparison circuit 14 is converted into a final output signal having a level of xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d (=VCC) by the transistor 15T in the output circuit 15 to be outputted from the output terminal 15Z.
Next, the operation of the conventional magnetic detector will be described with reference to FIGS. 6A to 6E. FIGS. 6A to 6E are timing charts showing the operation of the conventional magnetic detector. In FIGS. 6A to 6E, FIG. 6A shows the magnetic moving body 1, FIG. 6B shows magnetic fields applied to the magnetic resistance segments 2a, 2b, 2c and 2d, respectively, FIG. 6C shows resistance values of the magnetic resistance segments 2a to 2d, FIG. 6D shows an output signal of the differential amplification circuit 12, and FIG. 6E shows a final output signal.
The magnetic moving body 1 shown in FIGS. 4A and 4B is rotated about the rotational axis 4 to change the magnetic fields applied to the magnetic resistance segments 2a, 2b, 2c and 2d. Thus, as shown in FIGS. 6A and 6B, the magnetic fields applied to the magnetic resistance segments 2a to 2d are changed according to the shape of the magnetic moving body 1.
Furthermore, as shown in FIGS. 6C and 6D, the resistance values of the magnetic resistance segments 2a to 2d are changed due to the change in magnetic field, thereby obtaining the output signal of the differential amplification circuit 12. Then, as shown in FIG. 6E, the waveform of the output signal of the differential amplification circuit 12 is shaped by the comparison circuit 14, thereby being capable of obtaining the final output signal having the level xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d corresponding to the shape of the magnetic moving body 1.
In recent years, there has been made the demand for high resolution for realizing high performance even in magnetic detectors. However, the restrictions on the irregularity pitch for minimum detection, and the shape and processing of the magnetic moving body 1 in magnetic detectors place limitation on realizing the high resolution with the increase of the number of irregularities of the magnetic moving body 1.
Then, as an effective method for realizing the high resolution, there is a method of detecting a multipole-magnetized moving body as shown in FIGS. 7A and 7B.
FIGS. 7A and 7B are respectively a perspective view and a plan view each showing a construction of another conventional magnetic detector.
In FIGS. 7A and 7B, reference numeral 10 designates a multipole-magnetized moving body; reference numeral 2 designates a processing circuit portion in which a circuit is printed on a board; reference numerals 2a and 2d designate respectively magnetic resistance segments; reference numerals 2b and 2c designate respectively magnetic resistance segments; reference numeral 3 designates a magnet; and reference numeral 4 designates a rotational axis of the moving body 10. The rotational axis 4 is rotated so that the moving body 10 is also rotated synchronously therewith. Incidentally, for example, the magnetic resistance segments 2a and 2d are illustrated by one black block because the individual segments are so compacted that one segment can not be illustrated independently.
FIGS. 9A to 9E are timing charts showing the operation of another conventional magnetic detector shown in FIGS. 7A and 7B. In FIGS. 9A to 9E, FIG. 9A shows the moving body 10, FIG. 9B shows the magnetic fields applied to the magnetic resistance segments 2a, 2b, 2c and 2d, respectively, FIG. 9C shows the resistance values of the magnetic resistance segments 2a to 2d, FIG. 9D shows an output signal of the differential amplification circuit 12, and FIG. 9E shows a final output signal.
Now, the operating magnetic field range of the magnetic resistance element (constituted by the magnetic resistance segments 2a to 2d) becomes a problem. FIG. 8 is a graphical representation showing the operating magnetic field (MR loop characteristics) of the magnetic resistance element. In FIG. 8, the axis of abscissa represents the applied magnetic field (A/m), and the axis of ordinate represents the resistance change rate (%).
As shown in FIG. 8, since the resistance value (resistance change rate) of the magnetic resistance element becomes maximum with no magnetic field (applied magnetic field being zero) is applied thereto (when the magnitude of the applied magnetic field is zero), and decreases by application of the magnetic field irrespective of the direction, it is necessary to set the operating magnetic field range without crossing no magnetic field (zero magnetic field).
In the case of the conventional magnetic detector firstly described, the magnetic fields applied to the magnetic resistance element (constituted by the magnetic resistance segments 2a to 2d) are as shown in FIG. 6B. That is to say, the magnetic circuit is constructed such that when the magnetic resistance element faces the recess portion of the magnetic moving body 1, the nearly zero magnetic field is applied thereto, while when it faces the projection portion, the magnetic field is applied thereto.
For this reason, when detecting the multipole-magnetized moving body 11 as shown in FIGS. 7A and 7B which was secondly described, the magnetic fields applied to the magnetic resistance segments 2a to 2d will cross the zero magnetic field as shown in FIG. 9B. As a result, there has been a problem in that the magnetic resistance segments 2a, 2d and 2b, 2c show the similar resistance value change as shown in FIGS. 9C to 9E so that the output of the differential amplification circuit 12 is not obtained and moreover, the final output signal is not obtained.
In the light of the foregoing, the present invention has been made in order to solve the above-described problems associated with the prior art, and it is, therefore, an object of the present invention to provide a magnetic detector in which application of a bias magnetic field to a magnetic resistance element can prevent the magnetic field applied to the magnetic resistance element from crossing the zero magnetic field, and further, a multipole-magnetized moving body can be detected with high accuracy.
The present invention relates to a magnetic detector which includes a moving body, a magnetic resistance element, a processing circuit portion, and a magnet. The moving body is multipole-magnetized and rotated synchronously with a rotational axis. The magnetic resistance element detects a change in magnetic field of the rotating moving body. The processing circuit portion outputs a signal corresponding to the multipole magnetization of the moving body in accordance with the change in resistance value of the magnetic resistance element due to the change in magnetic field. The magnet applies a bias magnetic field to the magnetic resistance element. As a result, there is obtained an effect such that the magnetic field applied to the magnetic resistance element can be prevented from crossing the zero magnetic field, and thus, satisfactory detection can be detected.